Driveline vibration!!! I need help!

Hi everyone... the last 18 months I have been restoring my Dart GTS which had been setting since 1991 in my driveway...I've been trying to run down this drive train (I think) vibration the last 10 months... this is a 440 auto car ... engine, rebuilt... A727 tranny rebuilt, brakes rebuilt front to back.. front end rebuilt... drive shaft has new u joints and rebalanced (the drive shaft was 1.5 inches too short...lengthened drive shaft and still have vibration)...new wheels and tires...new motor and tranny mounts. checked tranny to differential angles as suggested by drive shaft company...still had vibration...checked if it was engine related by shifting down into 2nd gear and bringing up engine RPM to same RPM as vibration in 3rd gear(starts at 55 MPH 2800 RPM)...no vibration....this thing feels like a vibration coming from the mid section of the car to the rear end...I've had the differential covers opened up (its a Dana 60) and the differential inspected...needed a pinion shaft seal and 1 axle bearing...still have this vibration...Im at my wits end with this problem and I'm afraid to drive it at freeway speeds with this vibration...dont want to do any damage to the drivetrain...PLEASE I NEED HELP AND OR SUGGESTIONS...:axe: